Currently, most of the modern communication systems are based on fiber communication network, and fiber network has offered unprecedented huge capacity and installation flexibility and is able to support a variety of broadband applications that are under ceaseless development. Broadband and multi-channel tunable laser could help utilize the present fiber network redevices more efficiently. Data flow can be transferred from a congested channel to an unused channel by means of dynamic provision of broadband, thus Internet requirements are met. Use of a tunable laser makes rapid establishment or change of a light path easier, and it has become one of the important devices for implementing a dynamic fiber network.
In view of these applications, an ideal tunable laser shall have the following properties: wide tunable range covering C band and/or L band(approximately 1520 nanometers to 1620 nanometers), small size, fast tuning speed (sub-millisecond level) between the frequency intervals of any two international Telecommunication Unions (ITU) grid, excellent long-term working stability (service time over 25 years), high reliability under extreme environmental conditions, low power consumption and easy manufacturing and low cost.
With the successful development of Dense Wavelength Division Multiplexers (DWDM) and other high spectrum density related devices, the modern optical DWDM system has been developed with a frequency interval of 100 GHz, 50 GHz or 25 GHz and even higher density from the previous system with a frequency interval of 400 GHz and 200 GHz, Meanwhile, the transmission rate of an optical communication system has been increased to 10 Gbps, 40 Gbps or 100 Gbps from 2.5 Gbps in the past. This resulted in corresponding requirements on the small-size tunable laser for the optical communication system, and in particular, the stringent requirement on the optical frequency tuning interval and bandwidth of the tunable laser output. An external cavity tunable laser with high finesse etalons can reach the aforementioned requirements and accordingly becomes a good option for the new generation dynamic fiber optical communication system with high transmission rate and high spectrum density.
In an external cavity tunable laser, especially the lasers for fiber optical communication, the use of a high finesse etalon and a tunable narrow band optical filter will help to compress the laser output bandwidth and regulate the optical frequency tuning interval. If the frequency interval of laser output light is required to be Δf, the filtering bandwidth of the optical filter shall not exceed twice of this frequency interval, i.e. <2Δf, to avoid the laser working in a multi-mode state and therefore improve the working stability of the laser. If the frequency interval of laser output is required to be 50 GHz, the filtering bandwidth of the optical filter shall be less than 100 GHz; if the frequency interval of laser output light is required to be 25 GHz, the filtering bandwidth of the optical filter shall be less than 50 GHz. An optical filter such as a conventional optical grating filter or an acousto-optic filter with narrow bandwidth means higher manufacturing difficulty and higher cost, similarly, an etalon with narrow transmission spectrum interval means larger size, high manufacturing difficulty and cost.